Imprints of post-glacial climates and palaeogeography in the detrital clay mineral assemblages of an Upper Permian fluviolacustrine Gondwana deposit from northern Malawi

Abstract

Abstract Detailed quantitative clay mineral studies on an Upper Permian fluviolacustrine Gondwana deposit (≈55°S) from northern Malawi (9°S, 32°E) in central-southern Africa show imprints of Late Permian climates on the clay mineral assemblages. The clay mineral assemblage in the bottom 75 m is characterised by abundant illite, chlorite, regular smectite-chlorite (corrensite), regular illite-smectite, and vermiculite (Zone I). Between 75–240 m, illite and chlorite are the principal clay minerals, whereas corrensite and irregular illite-smectite mixed-layers disappear and vermiculite is rarely present (Zone II). The assemblage between 240–305 m is similar to that in Zone I, but is distinguished by the absence of irregular illite-smectite and a decrease in corrensite (Zone III). Kerogen fluorescence and chemistry, vitrinite reflectance, and palynomorph thermal alteration index indicate insignificant thermal diagenesis (50–70°C). The absence of depth-temperature related variation in mixed-layer mineral composition or in illite crystallinity indicate that the original clay mineral composition has not been overprinted by diagenetic modifications. Sandstone provenance and palaeocurrent directions suggest that detritus throughout deposition was derived from the slowly uplifting Nyika Plateau Basement and sedimentary rocks southwest and west of the basin. Furthermore, compositional and textural immaturity of the sandstones, as well as the fresh nature of the feldspars strongly suggest that the detritus was little-altered. The clay mineral assemblages in the three zones can thus be interpreted as representing the signatures of the climate during the periods of deposition. Zones I and III represent warmer and more seasonal climates than Zone II, which had generally more humid but less distinct seasons. Sedimentological features such as the presence of non-glacial rhythmites, sub-millimetric to millimetric laminated mudrocks, elevated CaCO 3 and TOC contents in claystones and mudstones in Zones I and III, whereas organic-lean, coarse siltstones and sandstones are common in Zone II. The three assemblage zones reflect changes perhaps associated with global Late Permian climate cycles. The quantitative clay mineral composition of the Upper Permian deposit is similar to those from modern temperate lake sediments from equivalent geographic and latitudinal settings, and source rocks in the drainage basins, in the northern hemisphere. The Upper Permian clay mineral assemblage contains substantial quantities of corrensite, which to date is documented from hypersaline or metamorphosed sedimentary environments. We report here its first occurrence from a shallow-buried freshwater lacustrine deposit and present evidence for syngenetic aggradation of detrital mixed-layers in a magnesium-enriched lacustrine depositional environment.